<!DOCTYPE html>
<p>There should be no red on this page.</p>
<div style="position:relative;">
    <div id="elm" style="width:200px; height:200px; background:red;">
        <div style="position:absolute; top:0; width:200px; height:200px; background:green;"></div>
    </div>
</div>
<script>
    document.body.offsetTop;
    document.getElementById("elm").style.marginLeft = "200px";
</script>
